1. Field of the Invention
The present invention relates to an electron beam lithography apparatus and an electron beam lithography method, and particularly to an electron beam lithography apparatus and an electron beam lithography method which are capable of performing highly-accurate exposure continuously for a long time after execution of calibration.
2. Description of the Prior Art
To improve throughput, an electron beam exposure apparatus, which is a representative example of an electron beam lithography apparatus, has a stencil mask with a variable rectangular aperture or multiple stencil mask patterns prepared, makes a pattern selection through beam deflection, and exposes a semiconductor wafer with an electron beam to transfer the pattern thereto.
As such an exposure apparatus, Japanese Patent Application Publication No. 2004-88071, for example, discloses an electron beam exposure apparatus which performs character projection exposure. In character projection exposure, a pattern region having a size of, for example, 20×20 μm is selected through beam deflection from multiple, for example one hundred, stencil patterns placed on a mask, and an electron beam is applied to the pattern region, so that a cross section of the beam is shaped into the form of the pattern. The beam having passed through the mask is deflected back by a deflector at a later stage, then is reduced at a certain reduction ratio determined by an electron optical system, for example, at a reduction ratio of 1/10, and is transferred to a sample such as a wafer. The area on the sample irradiated at once is, for example, 2×2 μm. If the stencil patterns on the mask are appropriately prepared according to a device pattern for exposure, having such multiple stencil patterns can contribute to a drastic reduction in the number of necessary exposure shots, and thus can improve throughput much higher than a case having only a variable rectangular aperture.
Further, there is proposed a multi-column electron beam exposure apparatus which includes multiple small-sized columns (referred to as column cells below) of such exposure apparatuses and performs exposure processing in parallel using the multiple column cells arranged above a wafer. Each column cell is equivalent to the column of an electron beam exposure apparatus with a single column. Since the multi-column electron beam exposure apparatus as a whole performs the exposure processing in parallel, exposure throughput can be multiplied by the number of columns.
Such an electron beam exposure apparatus has exposure data defining which pattern to use to expose which position on a sample. Signals to be applied to a deflector and a focus corrector for pattern exposure are determined according to the exposure data. Accurate electron-beam irradiation according to the exposure data is required as a precondition for improving throughput of the exposure processing of the electron beam exposure apparatus.
However, even when the electron-beam irradiation is performed by applying signals determined according to the exposure data to the deflector and the focus corrector, a phenomenon called a drift occurs in which the position of electron beam irradiation or the convergence (focus) by electromagnetic lenses changes with time.
This drift is affected strongly by the environment in which the exposure apparatus is used, particularly, by fluctuation in air pressure, and is thought to be very difficult to control. To solve this problem, Japanese Unexamined Patent Application Publication No. Sho 62-181426 describes a technique for making disturbance factors such as an air pressure and a temperature constant, the disturbance factors adversely affecting the optical performances in a projection optical apparatus.
Further, the electron beam exposure apparatus usually performs calibration at predetermined time intervals to keep the exposure accuracy high.
However, since the air pressure and the like change also after the calibration, it is difficult to perform highly-accurate exposure for a long time after the calibration. Moreover, frequent calibrations for keeping the exposure accuracy high lower the throughput of the exposure processing.